Field treatments used different plastic filters to screen UV-B (UV-B exclusion) or UV-B lamps (UV-B supplementation). Solar UV-B radiation was removed to produce the minus UV-B treatment using polyester filters (PE, 100 µm clear polyester plastic; Tap Plastics, Mountain View CA). This PE filter absorbs UV-B without significantly affecting UV-A or visible radiation (Casati and Walbot 2003). To control for differences in wind or humidity under plastic sheeting, cellulose acetate sheeting was placed over a control group (CA, 100 µm extra clear cellulose acetate plastic, Tap Plastics); the CA sheeting transmit most radiation from sunlight. Seeds from the six maize lines were planted during summers 2002 and 2003 outdoors at the Stanford Plant Growth Facility. Approximately 14 days after sowing and 7 days after germination, 1.5 m x 3.2 m of each plastic was draped over 1 m x 2.5 m wooden frames; the excess plastic was stapled to the sides of the frames to reduce light exposure. The N and S sides were left open to allow air circulation. However, 50 cm-long curtains with the same plastic were made on the E and W sides to avoid early morning and late afternoon UV exposure. The frames were maintained about 30 cm above the plant canopy during the course of the experiments. Temperature and humidity in the soil and in the leaves were recorded using an infrared thermometer (Model 210ALCS microcomputer based agri-term infrared thermometer, Everest Interscience Inc., Fullerton, CA) and a relative humidity hygrometer (Thermo-Hygro 800016, Sper Scientific, Scottsdale, AZ); there were no differences among the plots. Average canopy temperatures under the filters were always within 0.5 °C of each other, and in no case were consistent differences in temperature detected between filter treatments. The same was observed when humidity was recorded; differences were <25% between treatments. Measurements of incoming solar radiation were obtained using an Optronics model 752 spectroradiometer (Optronics Laboratories, Orlando, FL) that was calibrated against a National Bureau of Standards certified radiation source before each use. The spectrum under each filter was recorded periodically with 1 nm resolution across the entire sunlight spectrum (290 to 800 nm) to check for changes in the transmittance of the filters. Under our conditions, there were no significant reductions in transmittance after 3 weeks of exposure to solar UV-B, equivalent to the duration of our experiments. Consequently the filters were not replaced, but as they accumulate dust and could develop small tears, we cleaned them at least every fourth day and we replaced areas with small rips. Plants were grown under the specified conditions for 21 days, adult leaf samples from adult leaf 9 or 10 were collected for experiments at 4 PM in all treatments. The experiment was repeated using different replicate plots (with at least 10 plants of each genotype per plot) to control the field variables that might influence measurements. For experiments with supplemental UV-B radiation at the field, plants from the six lines were grown in the field for 4 weeks under natural sunlight conditions. After that, plants were illuminated by UV-B lamps using fixtures mounted 30 cm above the plants (Phillips, F40UVB 40 W and TL 20 W/12) for 8 h, and leaf samples were collected at 4 PM immediately after the end of the light treatment; this simulates a 4-fold increase in UV-B at 305 nm compared to the Stanford field in August at noon. The bulbs were covered using CA filters to exclude wavelengths lower than 280 nm. As a control, plants were exposed for the same period of time under the same lamps covered with PE (no UV-B treatment).